Shrouds are employed on some airfoils in gas turbine engines to prevent, during operation, twisting of the airfoils and to eliminate certain airfoil vibration modes. Generally, the shrouds extend laterally between adjacent rotor airfoils. Shrouds that extend laterally between the tips of adjacent airfoils are referred to as tip shrouds. Shrouds that are located intermediate of the airfoil dovetail and the airfoil tip are commonly referred to as mid-span shrouds. However, the term mid-span shroud has expanded to also include shrouds that are located anywhere along the airfoil span, not just at the midpoint span.
The shrouds extend laterally from the airfoil such that one shroud projects from the pressure surface and another shroud projects from the suction surface of each airfoil. When the engine is operating the suction surface shroud of each airfoil abuts the pressure surface shroud of the adjacent airfoil so that the shrouds define a shroud ring that provides support to the airfoils. The shroud ring resists vibration and twisting of the airfoils.
Shrouds were gradually eliminated from high thrust engine designs a few decades ago because the shrouds caused an efficiency loss in the fan rotor because they represent a source of drag in that the airflow is at rather high mach number and the flow around the shroud is diverted around the shroud. Consequently, modern fan rotors of turbofan engines with large diameters and high bypass ratios typically use wide chord airfoils without shrouds. However, eliminating the shrouds from the airfoils has negative consequences such as increased engine weight and increased fuel consumption. Without the shrouds, the engine design requires that the chord of the airfoil be increased and the airfoil count be reduced. These so-called wide chord airfoils without shrouds are typically fewer in number than the so-called narrow chord airfoils but each individual wide chord airfoil is much heavier so the net effect is an increase in engine weight. These changes also add engine length, fan airfoil weight that must be contained by a heavier fan airfoil containment system, and fan disk weight to manage the heavier fan airfoils and containment system weight. Additionally, use of the wide chord airfoil without the shroud that eliminated fan airfoil flutter may require the addition of a variable area fan nozzle to eliminate airfoil vibration, which has a very high system weight penalty.
Although eliminating the shrouds improves fan rotor efficiency, the extra weight added to the overall engine is a great detriment to fuel consumption efficiency. Thus, there is a need for a gas turbine engine which maintains aerodynamic efficiency of the fan airfoils and shrouds while also reducing engine weight.